In recent years, wireless data communication in domestic and enterprise environments have become increasingly commonplace and an increasing number of wireless communication systems have been designed and deployed. In particular, the use of wireless networking has become prevalent and wireless network standards such as IEEE 801.11a and IEEE 801.11 g have become commonplace.
The requirement for increasing data rates, communication capacity and quality of services has led to continued research and new techniques and standards being developed for wireless networking. One such standard is the IEEE 801.11n standard which is currently under development. IEEE 801.11n is expected to operate in the 2.4 GHz or 5 GHz frequency spectrum and promises data rates of around 100 Mbps and above on top of the MAC layer. IEEE 801.11n will use many techniques which are similar to the earlier developed IEEE 801.11a and IEEE 801.11g standards. The standard is to a large extent compatible with many of the characteristics of the earlier standards thereby allowing reuse of techniques and circuitry developed for these. For example, as in the previous standards IEEE 801.11a and IEEE 801.11g, IEEE 801.11n will use Orthogonal Frequency Division Multiplex (OFDM) modulation for transmission over the air interface.
The frame or packet formats employed by the IEEE 801.11 a/g/n standards can be understood with reference to the open system interconnection (OSI) model, which defines the application, presentation, session, transport, network, data link, and physical layers. The data link layer includes a logical link control (LLC) layer and a media access control layer. The MAC layer controls how to gain access to the network, and the LLC layer controls frame synchronization, flow control and error checking. The physical layer (PHY) transmits signals over the network. FIG. 1 shows the LLC, MAC and PHY layers along with the IEEE 801.11 a/g/n frames with which they are associated. As shown, each MAC service data unit (MSDU) or frame 11, received from a logic link control layer (LLC) 10, is appended with a MAC header and a frame check sequence (FCS) trailer, at the MAC layer 20, to form a MAC layer protocol data unit (MPDU) or frame 21. At the physical layer, the MPDU is received as a physical layer service data unit (PSDU) or frame 22. At the physical layer 30, a physical layer convergence procedure (PLCP) header, a PLCP preamble, and tail and pad bits are attached to the PSDU frame 22 to form a physical layer protocol data unit (PPDU) or frame 31 for transmission on the channel.
In order to improve efficiency and to achieve the high data rates, IEEE 801.11n is planned to introduce a number of advanced techniques. For example, IEEE 801.11n communication is expected to be typically based on a plurality of transmit and receive antennas. Furthermore, rather than merely providing diversity from spatially separated transmit antennas, IEEE 801.11n will utilise transmitters having at least partially separate transmit circuitry for each antenna thus allowing different sub-signals to be transmitted from each of the antennas. The receivers may receive signals from a plurality of receive antennas and may perform a joint detection taking into account the number and individual characteristics associated with each of the plurality of transmit antennas and receive antennas. Specifically, IEEE 801.11n has seen the introduction of a Multiple-Transmit-Multiple-Receive (MTMR) antenna concept which exploits Multiple-Input-Multiple-Output (MIMO) channel properties to improve performance and throughput. MIMO processing operates in conjunction with information located in a PPDU frame or packet.
One class of MTMR techniques that is specified in IEEE802.11n is spatial mapping, which is performed on a signal that has been divided into a number of different spatial streams. Spatial mapping techniques include direct mapping, cyclic shift diversity, beamforming and spatial expansion techniques. In spatial expansion, space expanded symbols are transmitted from spatially separate antennas. The spatial expansion provides separate space-time streams for each of the spatially separate antennas. More specifically, the spatial expansion or coding includes encoding a stream of symbols to provide separate encoded streams for separate antennas. Each encoded stream is distinct. For example, differential delays can be imposed upon different space-time streams by imposing different phase rotations on the samples of each of the streams.
Problems can arise with conventional spatial mapping techniques because each transmit antenna is provided with a separate, fixed one of the streams. For instance, if the stream transmitted from one of the antennas is sufficiently attenuated at the receiver so that packets in that stream are lost, there is no opportunity for the receiver to recover those packets from other transmit antennas. This situation may occur when the receiver, situated in a computer, personal digital assistant (PDA), router, base station, set top box, cellular telephone or the like, is subject to interference from nearby objects. This problem can be particularly acute for a receiver situated in a set top terminal, which is generally fixed in location and often surrounded by other equipment that can cause the stream from one or more of the antennas to be significantly attenuated.
One way to overcome this problem involves cycling the streams among the various antennas so that even if part of a packet in a particular stream is not successfully received from one transmit antenna, the other parts of this packet can be received from another one of the transmit antennas. Such a scheme has been proposed in “Improved STBC-SM Transmission Scheme”, Huawei Technologies, IEEE 802.11-07/0292r0, March 2007. In this scheme, each stream is sequentially sent to the different transmit antennas. Mathematically, this is accomplished by applying a temporal rotation to the different spatial streams. Unfortunately, this scheme imposes significant additional burdens on the receiver, substantially increasing its complexity and cost. In particular, since the mapping of the spatial streams onto the transmit antennas varies with time, the receiver needs to compute a new equalizer for each different permutation received from the transmit antennas. The equalizer is used to perform channel equalization on each of the subcarriers in the signal to mitigate signal distortions experienced by the streams during transmission. Since channel equalization is computationally expensive, the need to compute additional equalizers requires the receiver to perform additional computationally intensive tasks. For instance, if three transmit antennas are employed, the number of equalizers that need to be computed increases by a factor of 3 for each subcarrier in the stream.
Accordingly, it would be desirable to provide a method and apparatus for transmitting a wireless signal from a MIMO transmitter that can avoid the aforementioned problems without increasing the complexity of the receiver.